In recent years, power conservation of electric appliances has been demanded from a perspective of environmental protection. A power supply circuit is not an exception. Thus, power conservation of the power supply circuit used in an electric appliance has been in progress. However, reduction in power consumption in the power supply circuit causes degradation in the power supply response and a load response characteristic. In light of this, in a background constant-voltage circuit, a current amount passing through its differential amplifier circuit is automatically controlled such that the current amount is increased at such occasions as power-on when a difference between an output voltage and a reference voltage is relatively large, and the current amount is reduced when the output voltage is close to an expected value. Thereby, the background constant-voltage circuit reduces an amount of its consumption current as a whole, while maintaining its response performance.
To improve the load response characteristic, and particularly to cope with a rapid increase in a load current, another background constant-voltage circuit additionally includes a high-speed alternating-current amplifier circuit. Thus, the background constant-voltage circuit quickly sends a voltage corresponding to a change in the output voltage back to a control electrode of an output voltage control transistor. Accordingly, the background constant-voltage circuit consumes a relatively small amount of current, while maintaining a high-speed load response characteristic.
The background constant-voltage circuit includes an alternating-current amplifier circuit including an operational amplifier circuit for improving a response speed to the change in a load current. In this background constant-voltage circuit, an offset voltage is generated at one input terminal of the operational amplifier circuit to establish a dead-zone voltage responsive to the change in the output voltage. Further, the alternating-current amplifier circuit is operated only when the change in the output voltage exceeds a predetermined value. Thereby, unnecessary consumption of current is prevented.
However, the background constant-voltage circuit including the operational amplifier circuit is integrated on a semiconductor device. Thus, the offset voltage generated in the input circuit of the operational amplifier circuit substantially changes due to variations of semiconductor devices occurring in a manufacturing process. To secure the offset voltage at the minimum level required for generating the dead-zone voltage, therefore, a design value range of the offset voltage needs to be relatively large in consideration of the variations. Therefore, in a case in which the offset voltage is substantially increased, for example, variations in the output voltage needs to be relatively large so as to drive and operate the alternating-current amplifier circuit. As a result, the load response characteristic is not much improved.